Electricity is generally produced by power stations using a variety of fuels for producing the energy, such as gas, oil, coal, or lignite. Another means consists in using nuclear fuel to produce the heat that is then transformed into electrical energy in high pressure steam turbines.
Renewable energies are also known that contribute to a greatly varying extent to the production of electricity in various countries. Mention can be made, amongst other possibilities of: hydraulic energy from dams, wind turbines, water turbines that take energy from sea currents, and various devices that recover energy from the swell of the sea, or indeed from solar energy.
Although installations that produce electricity from fuel or from water retained by dams can operate continuously at nominal or maximum power during very long periods, renewable energies are characterized by operation that is intermittent, and incorporating renewable energy in a network generally serves to off-load only a fraction of conventional power stations, some of which can then be run at low power or even merely stopped, while waiting for a power demand from the network.
Various techniques are known that use turbines to produce mechanical energy from fuels, such as gas or oil, which consist in compressing an oxidizer, generally air, then mixing it with a fuel and causing it to burn in a combustion chamber, and finally directing it towards an expansion turbine in order to recover the mechanical energy that is produced, the compressor and the expansion turbine generally being mounted in line on a common shaft. In that type of rotary machine, maximum efficiency is sought, where maximum efficiency is a function of the inlet temperature to the expansion turbine. The limits on operating temperature are due to the temperature behavior of the blades in the expansion turbine, given the very aggressive nature of combustion gas at very high temperature, combustion gas generally being made up of CO2, of NOx, and of other compounds that are aggressive for the alloys used for making the blades of the turbine.
Steam turbines are also known that are used in nuclear power stations for converting the energy of water that has been taken to very high temperature in the form of steam, firstly into mechanical energy, and then into electrical energy within generators that are coupled to the end of the steam turbine shaft. Such steam turbines operate in a closed circuit for the heat-conveying fluid, water in the steam phase upstream from the turbine and liquid water downstream from said turbine.
Attempts have been made for a long time to store energy so as to have energy available almost instantaneously during consumption peaks. Numerous techniques have been developed, and mention can be made, amongst others, of electrical storage batteries, generally lead-acid batteries, or pumping water up to a dam at altitude, for the purpose of driving turbines during peaks in energy demand.
Storing energy in lead-acid batteries is a valid technique for small and medium capacities, but when it is necessary to store the equivalent of a nuclear power station unit, i.e. about 1200 megawatts (MW) over periods of 24 hours (h) or 36 h, the installations required become gigantic and completely unrealistic.
Although dams constitute an excellent means for storing energy, suitable sites are unfortunately limited in number, and in addition storing very large quantities of energy requires enormous quantities of water to be mobilized, which quantities then need to be taken from the quantities that are available, and subsequently they need to be released during periods when there is no need for such quantities of water, e.g. for irrigation, in which case the water is then more or less wasted. Nevertheless, several sites comprise a high reservoir and a low reservoir, generally large-capacity lakes, and when storing energy, the content of the low lake is pumped up to the high lake, to be available for driving a turbine in the opposite direction when consumption peaks require additional power to be delivered to the electricity network.
Another technique consists in storing energy in the form of compressed air, and then retransforming it into mechanical energy by piston engines, vane motors, or indeed turbines.
Patent WO 2005/108758 describes a technique of storing energy in the form of heat in an underground enclosure, the heat being generated by compressing air that is initially at atmospheric pressure and at ambient temperature, with the temperature within the underground storage being about 700° C. In that application, the gas, i.e. air, flows in an open circuit from the free atmosphere into the cavern during the storage stage, and then from the cavern to the free atmosphere during the energy return stage.
In another technical field, regenerators are commonly employed in industries that use fire, i.e. with blast furnaces, in the ceramics and terra cotta industries, in the glass-making and cement-making industries, which regenerators consist in sending hot burnt gas into large towers to heat refractory masses contained therein so as to recover the heat from the gas, before releasing said gas into the atmosphere. When the maximum temperature is reached within the refractory materials, the flow of hot gas is stopped and a reverse flow of cool air is passed through, which air becomes heated on making contact with the refractory materials prior to being directed to the inlets of furnaces, or to burners. Those arrangements enable heat losses within industrial processes that consume large amounts of energy to be reduced very significantly.
The problem posed is to store electrical energy from conventional power stations, such as coal, gas, oil, or indeed nuclear power stations, in order to be able to return the energy very quickly in large quantities to the electricity network during peak periods when energy demand exceeds production capacity.
Likewise, with renewable energies, such as wind turbines or sea water turbines, the problem is to be able to store large quantities of energy during periods of strong wind or current, said energy corresponding to surplus production, in order to return said energy during a stage in which production is insufficient, i.e. when the wind or the current does not enable the energy production level to be maintained at a minimum threshold.